The present invention relates to an optical recording and reproducing apparatus. More particularly, the invention concerns an optical recording and reproducing apparatus in which a laser beam is converged into a fine beam spot of about 1 .mu.m dia. and applied to an optical recording medium to record signals at a high density and to reproduce the recorded signal, and the recorded signal is erased as the medium is irradiated with another laser beam spot.
In a typical example of an optical recording apparatus of the type described, the laser beam spot of small diameter is applied to a rotating optical recording disk. The recording of signals is made at a high density by making use of the energy of the laser beam, the intensity of which is modulated by the signals to be recorded. On the other hand, the reproduction of the recorded signals is carried out by applying a laser beam of a constant intensity on the signal recording portions of the optical recording disk and detecting any change in the laser beam reflected or transmitted by the optical recording disk.
This type of optical recording and reproducing apparatus offers various advantages such as a high recording density, a low memory cost per bit, a high access speed and stable recording and reproduction without requiring direct contact between the optical head and the optical recording medium. Because of these advantages, this type of optical recording and reproducing apparatus has been expected to provide novel memory media in the future information society.
Two types of optical recording and reproducing methods are available: namely, the write-once type and the erasing type.
The write-once type method is further sorted into several types of methods such as a method in which the optical recording film is locally evaporated by the heat energy of the laser beam to form pits by means of which the signals are recorded and reproduced, a method in which the optical density of the recording film is locally changed by the energy of the applied beam to record and reproduce the signals, and so forth.
The erasing type method also can be sorted into several methods such as a method in which signals are recorded and reproduced by a cooperation between the heat effect of the laser beam and an external magnetic field, and a method which is a modification of the write-once type method making use of the optical density change wherein the optical density is reversibly changed by making use of only the heat energy of the laser beam.
The reversible change in the optical density can be effected by various methods by making a repeated use of a change of state of the recording film between the amorphous state and the crystalline state, between one amorphous state and another amorphous state which is stable, or a change in the size of crystal grains in an amorphous matrix.
The optical recording and reproducing apparatus of the invention makes use of the above-described reversible change in the optical density of the optical recording film. The principle of the invention will be explained briefly hereinunder, before turning to the description of the invention.
For an easier understanding, it is assumed here that the change in the optical density is attained by making use of the change of state between an amorphous state and the crystalline state of the medium.
Referring to FIG. 1, illustrating, a model of the transition between an amorphous state and the crystalline state of the medium, the recording film in the amorphous state represented by A exhibits a small reflection factor and a large light transmittance. Conversely, the reflection factor is large and the light transmittance is small when the recording film is in the crystalline state represented by C.
When a portion of the recording film in the amorphous state A shown in FIG. 1 is locally heated up to near or above the melting temperature and then gradually cooled, the state of this portion is changed from the amorphous state A into crystalline state C. Conversely, when the temperature of a portion of the recording film in the crystalline state is locally heated to near or above the melting point and then quenched, the state of this portion is changed from crystalline state C into amorphous state A.
A practical method of realizing the heating/quenching cycle and heating/slow cooling cycle will be explained hereinunder.
Referring to FIG. 2a, a substantially circular minute spot L of, for example, a laser beam is applied to a recording medium which moves in the direction of the arrow relatively to the beam spot. If the intensity of this beam spot L is increased momentarily to locally heat up the thin film, the temperature rise in this local portion is promptly diffused to the recording film and the substrate so as to realize the heating/quenching process.
On the other hand, when a beam spot M, elongated in the direction of movement of the recording medium indicated by the arrow, is applied as shown in FIG. 2B to the recording medium while its intensity is increased progressively or intermittently, the irradiated portion of the recording medium is heated and then cooled at a cooling rate much smaller than that in the case of FIG. 2A, thus realizing the heating/slow cooling process.
Thus, the heating/quenching process is attained by applying the fine beam spot to the recording film in the form of pulse and modulating the intensity of the beam as a function of time, whereas the heating/slow cooling process is obtained by applying, continuously or discontinuously, a beam spot elongated in the direction of movement of the recording medium.
FIG. 3 shows an example of an erasable optical recording and reproducing apparatus which operates in accordance with the principle explained hereinabove.
The apparatus shown in FIG. 3 is designed such that two beams are applied to a guide track 51 on an optical disk. As is well known, an optical recording thin film is applied to the optical recording disk. An arrow A represents the direction of movement of the optical recording medium relatively to the beam spots M and L, while X represents a single point on the optical recording medium. The signal which has been recorded on the point X is erased when scanned by the elongated beam spot M or a new signal is recorded and reproduced when the same portion is scanned by the circular beam spot L. FIG. 4 shows examples of intensity distribution profiles of the beam spots M and L shown in FIG. 3. In this Figure, r.sub.m represents the optical axis of the beam spot M. The beam intensity is distributed around the optical axis r.sub.m substantially in the form of Gaussian beam such as to form an elongaged beam spot along the guide track 51. Similarly, the beam intensity for forming the beam spot L is distributed in the form of Gaussian beam about the optical axis r.sub.1 such as to form the circular beam on the guide track 51. In consequence, the signal recorded on the point X is erased when the point X is heated and then slowly cooled by the application of the beam spot M. Then, as the point X is heated and then quenched by the application of the beam spot L, a new signal is recorded. The recording and erasing of the signal are thus performed. This method is advantageous in that it permits recording and erasing in real time with a simple arrangement but encounters a problem in that the laser beam has to be elongaged in order to hold the medium at the temperature necessary for the crystallization during the erasing. Therefore, for the purpose of obtaining a beam power density sufficient for the temperature rise, it is necessary to employ a laser of large power.
From FIG. 4, it will be clear also that the temperature of the point X approaches the melting point only after it has been accessed by the optical axis r.sub.m of the erasing beam spot M. Therefore, only the left half part of the optical spot M is utilized for the slow cooling of the heated portion of the recording medium. Thus, the length of the beam spot M along the guide track has to be further increased, in order to attain a time long enough for allowing the crystallization, requiring a further increase in the laser power. Thus, the described method encounters a problem in that the independent control of the power level and the length of the erasing spot M is often prohibited.